Nakhlite and chassignite martian meteorites have similar crystallization (1340 ± 40 Ma) and ejection (∼11 Ma) ages, and 87Rb-87Sr and 143Sm-144Nd compositions. Using a near-comprehensive suite of these rocks, we place further constraints on nakhlite and chassignite petrogenesis, utilizing bulk rock and mineral major- and trace-element compositions, and quantitative textural data for 17 samples, including three recent finds (Northwest Africa [NWA] 10153, NWA 10645, and NWA 11013). Bulk rock and mineral compositions indicate that nakhlites and chassignites originated from <5% partial melting of a highly depleted source, in the presence of residual garnet. Significant fractionation of olivine and pyroxene from parental magmas led to formation of cumulate dunites (chassignites), and augite-rich cumulates with relatively low abundances of interstitial material (nakhlites). We show that two nakhlite groups exist with high and low absolute trace-element abundances, which are consistent with groupings from previous studies based on mesostasis content and volatile element contents. The discrepancy between the parental melt and cumulate bulk rock compositions indicates that a missing fractionated melt composition complementary to nakhlites and chassignites should exist on Mars. Quantitative textural analyses of both nakhlites and chassignites are consistent with emplacement as distinct lava flows and/or magmatic bodies close to the martian surface, rather than from a single sill or lava flow sequence. Although originating from the same parental melt to nakhlites, chassignites likely represent cumulates that were either erupted as xenoliths, or occurred as crystal settling pods within dikes or sills and thus represent a different batch of flow/magma from the nakhlites. Determination of an ancient 207Pb-206Pb age (3.95 ± 0.16 Ga) for an apatite grain in NWA 998 is consistent with hydrothermal alteration of nakhlites by ancient crustal-derived fluids immediately following their emplacement. We interpret the apatite age, which is highly distinct from the crystallization age of nakhlites, to indicate addition of Cl-rich fluids driven by hydrothermal circulation of martian crustal brines during emplacement of the nakhlites and chassignites. Although the spatial location of nakhlites and chassignites at the martian surface remains unconstrained, our results indicate similar emplacement features to those observed in terrestrial volcano-magmatic systems.

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit He-3/He-4 ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High He-3/He-4 ratios must indicate derivation from a mantle source with high time-integrated He-3/(U+Th) relative to depleted MORB-source mantle. The location of the high He-3/He-4 mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earth's mantle. It has been proposed that the high He-3/He-4 reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have He-3/He-4 ratios more radiogenic than MORB, strongly arguing against a widespread high He-3/He-4 source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high He-3/He-4 in any samples known to originate from this environment. Therefore, volcanic rocks with He-3/He-4 greater than MORB He-3/He-4 are likely to sample a mantle source with high time-integrated He-3/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the He-3/He-4 distribution in MORB.

The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favoured due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have 186Os ([{(186Os/188Ossample[t]/186Os/188OsPM(t)) -1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ~2.7 Ga Stillwater Igneous Complex, -0.05 to 0.17 for the ~1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ~0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modelled through high sulfide-melt partitioning (typically >8000) and emphasise the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R-factor), it is possible to explain the variations in 186Os-187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that <4% of ancient high Pt/Os crustal contributions can explain the composition of the chromitites and peridotites that were examined. Our observations are consistent with published models for chromitite genesis that invoke either crustal melt-primitive melt mixing, or cumulate assimilation. A crustal origin for radiogenic 186Os is a possible cause for 186Os/188Os ratio variations observed in some komatiites. It is more difficult to explain radiogenic 186Os/188Os measured in Hawaiian lavas by crustal contamination processes. Instead, ancient high Pt/Os oceanic crust, shallow mantle sources such as metasomatic sulfide, or metal-rich large low-shear wave velocity provinces at the core-mantle boundary, all remain valid explanations.

Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials(1,2). However, the residence time of these subducted materials in the mantle is uncertain and model-dependent(3), and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust(3,4). Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago(5-7). Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur-probably derived from hydrothermally altered oceanic crust-was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Delta S-33 values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions(8,9). This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.

Cryptic colouration in crustaceans, important for both camouflage and visual communication, is achieved through physiological and morphological mechanisms that are sensitive to changes in environmental conditions. Consequently, ocean warming and ocean acidification can affect crustaceans’ biophotonic appearance and exoskeleton composition in ways that might disrupt colouration and transparency. In the present study, we measured growth, mineralization, transparency, and spectral reflectance (colouration) of the caridean grass shrimp Hippolyte californiensis in response to pH and temperature stressors. Shrimp were exposed to ambient pH and temperature (pH 8.0, 17 °C), decreased pH (pH 7.5, 17 °C), and decreased pH/increased temperature (pH 7.5, 19 °C) conditions for 7 weeks. There were no differences in either Mg or Ca content in the exoskeleton across treatments nor in the transparency and spectral reflectance. There was a small but significant increase in percent growth in the carapace length of shrimp exposed to decreased pH/increased temperature. Overall, these findings suggest that growth, calcification, and colour of H. californiensis are unaffected by decreases of 0.5 pH units. This tolerance might stem from adaptation to the highly variable pH environment that these grass shrimp inhabit, highlighting the multifarious responses to ocean acidification, within the Crustacea.

The sensitivity of trace element concentrations to processes governing solid-melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism, and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta, and Nb (TITAN) anomalies to isotopic tracers, such as 3He/4He and 187Os/188Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti* = 0.28–2.35), Ta (Ta/Ta* = 0.11–93.4), and Nb (Nb/Nb* = 0.13–17.8) anomalies that show negligible correlations with 3He/4He ratios, indicating that TITAN anomalies are not derived from the less-degassed mantle source traced by high-3He/4He. Positive TITAN anomalies can be modeled using variable degrees (0.1–10%) of nonmodal batch partial melting of garnet-spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid-ocean ridge basalt-like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle-normalized spider diagrams. Because partial melting and AFC are common to all mantle-derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.

Arculus et al.1 raise an important question regarding the use of terrestrial rock nomenclature to characterize extraterrestrial materials. Here the issue relates to the definition of ‘andesite’, and whether the felsic achondrite meteorites GRA 06128 and GRA 06129 (GRA 06128/9) can and should be classified using this term2. More broadly, the question is whether extraterrestrial rocks should be classified using standard petrologic and geochemical criteria, such as mineralogy and major-element bulk composition, developed for the description of terrestrial rocks3,4. The approach of Arculus et al.1 is that petrogenetic process is of equal or greater importance when classifying a rock. This question must ultimately be decided by the scientific community.

Organic carbon-rich sediment assimilation by basaltic magmas leads to enhanced emission of greenhouse gases during continental flood basalt eruptions. A collateral effect of these interactions is the generation of low oxygen fugacities (fO2)(below the iron-wüstite [IW] buffer curve) during magmatic crystallization, resulting in the precipitation of native-iron. The occurrence of native-iron bearing terrestrial basaltic rocks are rare, having been identified at three locations: Siberia, West Greenland, and Central Germany. We report the first combined study of Re–Os isotopes, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), and trace-element abundances for these three occurrences, in addition to host sediments at West Greenland. To quantify the amount of crustal assimilation experienced by the magmas, we present combined crystallization and assimilation models, together with fractional crystallization models, to assess how relative abundances of the HSE have been modified during crystallization. The radiogenic osmium isotopic compositions (γOsinitial +15 to +193) of mafic igneous samples are consistent with assimilation of old high Re/Os crustal contaminants with radiogenic 187Os/188Os, whereas the HSE inter-element fractionations (Pd/Os 2 to >10,000) suggest that some Siberian samples underwent an early stage of sulfide removal.

Metalliferous samples from the Siberian intrusions of Khungtukun and Dzhaltul (associated with the Siberian flood basalts) yield internal 187Re–187Os ages of 266 ±83 Ma and 249 ±50 Ma, respectively, reflecting late-Permian emplacement ages. These results imply that crustal assimilation took place prior to crystallization of native-Fe. In contrast, metalliferous samples from Disko Island and Bühl (associated with the West Greenland flood basalts, and the Central European Volcanic Province, respectively) have trends in 187Re/188Os–187Os/188Os space corresponding to apparent ages older than their reported crystallization ages. These anomalous ages probably reflect concurrent assimilation of high Re/Os, radiogenic 187Os crust during crystallization of native-Fe, consistent with the character of local West Greenland sediments. In all three locations, calculations of combined assimilation of crustal sediments and fractional crystallization indicate between 1–7% assimilation can account for the Os-isotope systematics. In the case of Siberian samples, incompatible trace-element abundances indicate that lower crustal assimilation may have also occurred, consistent with the suggestion that crustal assimilation took place prior to native-Fe precipitation. The extent of local crustal contamination at Siberia, West Greenland, and Bühl necessitates that significant quantities of CH4, CO, CO2, SO2and H2O were released during assimilation of carbonaceous sediments. Consequently, carbonaceous sediment–basalt melt interactions have collateral effects on total gas output from flood basalt volcanism into the atmosphere. However, the amount of carbonaceous sediment contamination experienced by melts forming the Khungtukun and Dzhaltul intrusions alone, cannot explain the major C-isotope excursions at the Permo–Triassic mass-extinction event. Instead, further unsampled intrusions that also experienced significant carbonaceous sediment–melt interactions would be required. Enhanced greenhouse gas-emission during the Permo–Triassic mass extinction may have been facilitated by a combination of mantle melting and carbonaceous sediment–melt interactions, together with other proposed mechanisms, including wildfires, or by microbial metabolic exhalation.

Ureilite meteorites are partially melted asteroidal-peridotite residues, or more rarely, cumulates that can contain greater than three weight percent carbon. Here we describe an exceptional C-rich lithology, composed of 34 modal% large (up to 0.8 mm long) crystalline graphite grains, in the Antarctic ureilite meteorite Miller Range (MIL) 091004. This C-rich lithology is embedded within a silicate region composed dominantly of granular olivine with lesser quantities of low-Ca pyroxene, and minor FeNi metal, high-Ca pyroxene, spinel, schreibersite and troilite. Petrological evidence indicates that the graphite was added after formation of the silicate region and melt depletion. Associated with graphite is localized reduction of host olivine (Fo88-89) to nearly pure forsterite (Fo99), which is associated with FeNi metal grains containing up to 11 wt.% Si. The main silicate region is typical of ureilite composition, with highly siderophile element (HSE) abundances ∼0.3 × chondrite, 187Os/188Os of 0.1260 to 0.1262 and Δ17O of -0.81 ±0.16‰. Mineral trace-element analyses reveal that the rare earth elements (REE) and the HSE are controlled by pyroxene and FeNi metal phases in the meteorite, respectively. Modelling of bulk-rock REE and HSE abundances indicates that the main silicate region experienced ∼6% silicate and >50% sulfide melt extraction, which is at the lower end of partial melt removal estimated for ureilites. Miller Range 091004 demonstrates heterogeneous distribution of carbon at centimeter scales and a limited range in Mg/(Mg+Fe) compositions of silicate grain cores, despite significant quantities of carbon. These observations demonstrate that silicate rim reduction was a rapid disequilibrium process, and came after silicate and sulfide melt removal in MIL 091004. The petrography and mineral chemistry of MIL 091004 is permissive of the graphite representing late-stage C-rich melt that pervaded silicates, or carbon that acted as a lubricant during anatexis and impact disruption in the parent body. Positive correlation of Pt/Os ratios with olivine core compositions, but a wide range of oxygen isotope compositions, indicates that ureilites formed from a compositionally heterogeneous parent body that experienced variable sulfide and metal melt-loss that is most pronounced in relatively oxidized ureilites with Δ17O between -1.5 and ∼0‰.

The timing, causes and extent of mantle heterogeneity preserved in the ∼492 Ma Shetland Ophiolite Complex (Scotland) are evaluated using Re–Os isotope and whole rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance measurements of a suite of eight chromitites and 21 serpentinised harzburgites and dunites. Shetland dunites have more variable initial 187Os/188Os, as well as absolute and relative abundances of the HSE, compared to spatially associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (γOs492Ma of −5.3 to +2.6) preserve evidence for a Mesoproterozoic depletion event, but are dominated by contemporary chondritic, ambient upper mantle compositions. The dunites have γOs492Ma values ranging between −3.3 and +12.4, reflecting dunite formation by higher degrees of melt interaction with mantle rock than for the spatially associated harzburgites.

Chromitite seams from three locations separated by <500 m have a large range in HSE concentrations (e.g., 0.09 to ∼2.9 μg g−1 Os) with initial γOs492Ma values ranging only from +0.48 to +3.95. Sulphides, arsenides and platinum-group minerals are the primary hosts for the HSE in the chromitites. Their isotopic variations reflect initial isotopic heterogeneity in their primary magmatic signatures. Coupled with field observations that support chromitite formation in concentrated zones of enhanced melt flow, the isotopic dichotomy between the harzburgites and the chromitites suggests that chromitite 187Os/188Os compositions may better approximate the upper limit, rather than an average value, of the bulk convecting upper mantle.

The Shetland peridotite compositions reflect protracted melt depletion (low-Al2O3) and melt percolation events in a supra-subduction zone (SSZ) setting at ∼492 Ma, following an earlier (Mesoproterozoic) melt-depletion event. These results provide further evidence that ancient chemical complexities can be preserved in the upper mantle during ocean plate formation. Chromitites and peridotites from the Shetland Ophiolite Complex also attest to lithological and geochemical heterogeneities generated at scales of less than tens of metres during the formation of ancient oceanic lithosphere by high-degree SSZ melt extraction, percolation and during chromitite formation in the oceanic lithosphere.

Mars is considered to have formed as a planetary embryo that experienced extensive differentiation early in its history. Shergottite meteorites preserve evidence for this history, and for late accretion events that affected their mantle sources within Mars. Here we report the first coupled 187Re–187Os, 87Sr/86Sr, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) and major element abundance dataset for martian shergottites that span a range of MgO contents, from 6.4 to 30.3 wt.%. The shergottites range from picro-basalt to basaltic-andesite compositions, have enriched to depleted incompatible trace-element compositions, and define fractional crystallization trends, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≫ Pt ≥ Pd ≥ Re. This order of compatibility is like that defined previously for Earth and the Moon, but the fractionation of strongly compatible Os, Ir and Ru appears to take place at higher MgO contents in martian magmas, due to early onset of sulfide fractionation. In general, enriched shergottites have lower MgO contents than intermediate or depleted shergottites and have fractionated HSE patterns (Re + Pd + Pt > Ru + Ir + Os) and more radiogenic measured 87Sr/86Sr (0.7127–0.7235) and 187Os/188Os (0.140–0.247) than intermediate or depleted shergottite meteorites (87Sr/86Sr = 0.7010–0.7132; 187Os/188Os = 0.127–0.141). Osmium isotope compositions, corrected for crystallization age, define compositions that are implausibly unradiogenic in some enriched shergottites, implying recent mobilization of Re in some samples. Filtering for the effects of alteration and high Re/Os through crystal-liquid fractionation leads to a positive correlation between age-corrected Sr and Os isotope compositions. Mixing between hypothetical martian crustal and mantle reservoirs are unable to generate the observed Sr–Os isotope compositions of shergottites, which require either distinct and discrete long-term incompatible-element depleted and enriched mantle sources, or originate from hybridized melting of deep melts with metasomatized martian lithosphere. Using MgO-regression methods, we obtain a modified estimate of the bulk silicate Mars HSE composition of (in ng g−1) 0.4 [Re], 7.4 [Pd], 9.6 [Pt], 6.2 [Ru], 3.7 [Ir], 4 [Os], and a long-term chondritic 187Os/188Os ratio (∼0.1312). This result does not permit existing models invoking high-pressure and temperature partitioning of the HSE. Instead, our estimate implies 0.6–0.7% by mass of late accretion of broadly chondritic material to Mars. Our results indicate that Mars could have accreted earlier than Earth, but that disproportional accretion of large bodies and a relative constant flux of accretion of available materials in the first 50–100 Ma of Solar System led to the broad similarity in HSE abundances between Earth and Mars.

New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo(64-2)) and pyroxene (Fs(32)Wo(8)En(60) to Fs(84-86)Wo(15)En(2-0)) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (similar to 2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon. (c) 2005 Elsevier Inc. All rights reserved.

Olivine-dominated (70-80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO-rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Delta O-17 value of -1.19 +/- 0.10 parts per thousand, and low bulk-rock Mg/(Mg+Fe) (0.39), similar to the FeO-rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk-rock major-, minor-, and trace-element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and Os-187/Os-188 (0.1262), implies a FeO-and volatile-rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe-Ni-S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal-rich partial melts from FeO-rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to << 5%). As such, LEW 88763 represents the least-modified FeO-rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe, Ni-FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO-rich meteorites, such as brachinites, brachinite-like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.

Mechanisms for the formation of crust on planetary bodies remain poorly understood(1). It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics(1,2), whereas the Moon acquired its feldspar- rich crust by way of plagioclase flotation in a magma ocean(3,4). Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large- scale differentiation processes(5). Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak ( GRA) 06128 and GRA 06129. These meteorites are feldspar- rich, with andesite bulk compositions. Their age of 4.5+/-0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide- rich nature of the meteorites are most consistent with an origin as partial melts from a volatile- rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early- formed materials in the Solar System.

Ultramafic cumulate rocks form during intrusive crystallization of high-MgO magmas, incorporating relatively high abundances of compatible elements, including Cr and Ni, and high abundances of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re). Here, we utilize a suite of cumulate xenoliths from Piton de la Fournaise, La Réunion (Indian Ocean), to examine the mantle source composition of the Réunion hotspot using HSE abundances and Os isotopes. Dunite and wherlite xenoliths and associated lavas from the Piton de la Fournaise volcanic complex span a range of MgO contents (46 to 7 wt.%), yet exhibit remarkably homogeneous 187Os/188Os (0.1324±0.0014, 2σ), representing the Os-isotopic composition of Réunion hotspot primary melts. A significant fraction of the xenoliths also have primitive upper-mantle (PUM) normalized HSE patterns with elevated Ru and Pd (PUM-normalized Ru/Ir and Pd/Ir of 0.8–6.3 and 0.2–7.2, respectively). These patterns are not artifacts of alteration, fractional crystallization, or partial melting processes, but rather require a primary magma with similar relative enrichments. Some highly olivine-phyric (>40 modal percent olivine) Piton de la Fournaise lavas also preserve these relative Ru and Pd enrichments, while others preserve a pattern that is likely related to sulfur saturation in evolved melts.

The estimate of HSE abundances in PUM indicates high Ru/Ir and Pd/Pt values relative to carbonaceous, ordinary and enstatite chondrite meteorite groups. Thus, the existence of cumulate rocks with even more fractionated HSE patterns relative to PUM suggests that the Réunion hotspot samples a yet unrecognized mantle source. The origin of fractionated HSE patterns in Réunion melts may arise from sampling of a mantle source that experienced limited late accretion (<0.2% by mass) compared with PUM (0.5–0.8%), possibly involving impactors that were distinct from present-day chondrites, or limited core–mantle interactions. Given the remarkably homogeneous Os, Pb, and noble-gas isotopic signatures of Réunion, which plot near the convergence point of isotopic data for many hotspots, such a conclusion provides evidence for an early differentiated and subsequently isolated mantle domain that may be partially sampled by some ocean island basalts.

In order to establish the role and expression of silicate-metal fractionation in early planetesimal bodies, we have conducted a highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and 187Re-187Os study of acapulcoite-lodranite meteorites. These data are reported with new petrography, mineral chemistry, bulk-rock major and trace element geochemistry, and oxygen isotopes for Acapulco, Allan Hills (ALHA) 81187, Meteorite Hills (MET) 01195, Northwest Africa (NWA) 2871, NWA 4833, NWA 4875, NWA 7474 and two examples of transitional acapulcoite-lodranites, Elephant Moraine (EET) 84302 and Graves Nunataks (GRA) 95209. These data support previous studies that indicate that these meteorites are linked to the same parent body and exhibit limited degrees (<2–7%) of silicate melt removal. New HSE and osmium isotope data demonstrate broadly chondritic relative and absolute abundances of these elements in acapulcoites, lower absolute abundances in lodranites and elevated (>2 × CI chondrite) HSE abundances in transitional acapulcoite-lodranite meteorites (EET 84302, GRA 95209). All of the meteorites have chondritic Re/Os with measured 187Os/188Os ratios of 0.1271 ± 0.0040 (2 St. Dev.). These geochemical characteristics imply that the precursor material of the acapulcoites and lodranites was broadly chondritic in composition, and were then heated and subject to melting of metal and sulfide in the Fe-Ni-S system. This resulted in metallic melt removal and accumulation to form lodranites and transitional acapulcoite-lodranites. There is considerable variation in the absolute abundances of the HSE, both among samples and between aliquots of the same sample, consistent with both inhomogeneous distribution of HSE-rich metal, and of heterogeneous melting and incomplete mixing of silicate material within the acapulcoite-lodranite parent body. Oxygen isotope data for acapulcoite-lodranites are also consistent with inhomogeneous melting and mixing of accreted components from different nebular sources, and do not form a well-defined mass-dependent fractionation line. Modeling of HSE inter-element fractionation suggests a continuum of melting in the Fe-Ni-S system and partitioning between solid metal and sulfur-bearing mineral melt, where lower S contents in the melt resulted in lower Pt/Os and Pd/Os ratios, as observed in lodranites. The transitional meteorites, EET 84302 and GRA 95209, exhibit the most elevated HSE abundances and do not follow modelled Pt/Os and Pd/Os solid metal-liquid metal partitioning trends. We interpret this to reflect metal melt pooling into domains that were sampled by these meteorites, suggesting that they may originate from deeper within the acapulcoite-lodranite parent body, perhaps close to a pooled metallic ‘core’ region. Petrographic examination of transitional samples reveals the most extensive melting, pooling and networking of metal among the acapulcoite-lodranite meteorites. Overall, our results show that solid metal-liquid metal partitioning in the Fe-Ni-S system in primitive achondrites follows a predictable sequence of limited partial melting and metal melt pooling that can lead to significant HSE inter-element fractionation effects in proto-planetary materials.

The Moon is depleted in volatile elements relative to the Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high mu (238U/204Pb) and long-term Rb/Sr depletion are distinguishing features of the Moon, relative to the Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and planets and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile loss through localized eruptive degassing are not consistent with the available S, Cl, Zn and K isotopes and abundance data for the Moon. The most probable cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, in this model, the volatile loss requires preferential delivery and retention of late-accreted volatiles to the Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately volatile elements (e.g. Pb, Zn; 5 to >10%) relative to highly siderophile elements (approx. 0.5%) for the Earth. Models of early magma ocean phases may be more effective in explaining the volatile loss. Basaltic materials (e.g. eucrites and angrites) from highly differentiated airless asteroids are volatile-depleted, like the Moon, whereas the Earth and Mars have proportionally greater volatile contents. Parent-body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.

Volatile element and compound abundances vary widely in planets and were set during the earliest stages of solar system evolution. Experiments or natural analogs approximating these early conditions are limited. Using silicate glass formed from arkosic sands during the first nuclear detonation at the Trinity test site, New Mexico, we show that the isotopes of zinc were fractionated during evaporation. The green silicate glasses, termed “trinitite,” show +0.5 ± 0.1‰/atomic mass unit isotopic fractionation from ~200 m to within 10 m of ground zero of the detonation, corresponding to an α fractionation factor between 0.999 and 0.9995. These results confirm that Zn isotopic fractionation occurs through evaporation processes at high temperatures. Evidence for similar fractionations in lunar samples consequently implies a volatile-depleted bulk Moon, with evaporation occurring during a giant impact or in a magma ocean.

Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo(77-73)); Type-II inclusions occur in phenocryst mantles (Fo(71-66)); Type-III inclusions occur in phenocryst rims (Fo(61-51)) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (similar to 150-250 mu m diameter) to Type-II (similar to 100 mu m diameter) to Type-III (similar to 25-75 mu m diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo(77) composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (similar to 10 wt%). This is consistent with a calculation indicating that similar to 10 wt% agnesian (Fo(77-73)) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo(77). Thus, two independent estimates indicate that LAR 06319 contains similar to 10 wt% cumulate olivine. The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major-and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites. (C) 2011 Elsevier Ltd. All rights reserved.

Shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle. In this study, a suite of 23 shergottites, spanning their known range in bulk compositions, Rb-Sr, Sm-Nd, and Lu-Hf isotopes, were measured for Re-187-Os-187 isotopic systematics and highly siderophile element abundances (HSE: including Os, Ir, Ru, Pt, Pd, Re). The chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long-term HSE budget of its derivative melts. Possible effects upon HSEs related to crustal contamination, as well as terrestrial and/or martian surface alteration are also examined. Some of the shergottites are hot arid-desert finds. Their respective acetic acid leachates and residues show that both Re and Os display open-system behavior during sample residence at or near the martian and/or terrestrial surfaces. In some meteorites, the alteration effects can be circumvented by analysis of the leached residues. For those shergottites believed to record robust Re-Os isotopic systematics, calculated initial Os-187/Os-188 are well correlated with the initial Nd-143/Nd-144. Shergottites from mantle sources with long-term melt-depleted characteristics (initial epsilon Nd-143 of + 36 to + 40) have chondritic initial gamma Os-187 ranging from -0.5 to + 2.5. Shergottites with intermediate initial epsilon Nd-143 of + 8 to + 17 have a range in initial gamma Os-187 of -0.6 to + 2.3, which overlaps the range for depleted shergottites. Shergottites from long-term enriched sources, with initial epsilon Nd-143 of similar to-7, are characterized by suprachondritic gamma Os-187 values of + 5 to + 15. The initial gamma Os-187 variations for the shergottites do not show a correlation with indices of magmatic differentiation, such as MgO, or any systematic differences between hot arid-desert finds, Antarctic finds, or observed falls. The strong correlation between the initial epsilon Nd-143 and gamma Os-187 in shergottites from approximately + 40 and 0 to -7 and + 15, respectively, is assessed in models for mixing depleted mantle-derived melts with ancient crust (modeled to be similar to evolved shergottite in composition), and with assimilation-fractional crystallization. These models show that the correlation is unlikely to result from participation of martian crust. More likely, this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca. 4.5 Ga. These models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean. The expanded database for the HSE abundances in shergottites suggests that their martian mantle sources have similar HSE abundances to the terrestrial mantle, consistent with prior studies. The relatively high HSE abundances in both planetary mantles likely cannot be accounted for by high pressure-temperature metal-silicate partitioning at the bases of magma oceans, as has been suggested for Earth. If the HSE were instead supplied by late accretion, this event must have occurred prior to the crystallization of the last martian magma ocean. (C) 2011 Elsevier Ltd. All rights reserved.

Antarctic lunar meteorites Meteorite Hills 01210 and Pecora Escarpment 02007 are breccias that come from different regolith lithologies on the Moon. MET 01210 is composed predominantly of fractionated low-Ti basaltic material and is classified as an immature, predominantly basaltic glassy matrix regolith breccia. PCA 02007 is a predominantly feldspathic regolith breccia consisting of metamorphosed feldspathic, noritic, troctolitic and noritic-anorthosite clasts, agglutinate and impact-glasses, as well as a number of basaltic clasts with mare and possible non-mare affinities. The basalt clasts in MET 0 12 10 have undergone 'Fenner' trend enrichments in iron and may also have witnessed late-stage crystallization of zircon or a zirconium-rich mineral. Some of the features of MET 0 1210 are similar to other basaltic lunar breccia meteorites (e.g., Northwest Africa 773; Elephant Moraine 87521/96008; Yamato 793274/981031), but it is not paired with them. The presence of metamorphic anorthositic clasts as well as agglutinates indicates a small regolith component. Similarities with previously discovered evolved (e.g., LaPaz Icefield 02205; Northwest Africa 032) and ferroan (e.g., Asuka 881757; Yamato 793169) basaltic lunar meteorites suggest a similar mare source region for MET 01210. Despite lack of evidence for pairing, PCA 02007 shares many features with other feldspathic regolith breccias (e.g., Yamato 791197, Queen Alexandra Range 94281), including a high Mg/Fe whole-rock composition, glass spherules, agglutinate fragments and a diverse clast inventory spanning the range of ferroan anorthosite and high magnesium suite rocks. Some of the basalt fragments in this sample are fractionated and have an igneous origin. However, the majority of the basalt fragments are impact melt clasts. PCA 02007 supports previous studies of feldspathic lunar meteorites that have suggested an aluminous crust for the Moon, with compositions more similar to magnesium granulite breccias than ferroan anorthosites. A 'chondrule-like' fragment found in PCA 02007 and unlike any previously described lunar material is described and tentatively identified as the remnants of a chondritic lunar impactor. This clast is porphyritic with equant olivines that have forsterite-rich cores (Fo(> 98)), extreme normal zonation to more fayalitic rims (Fo(> 44)), and a mineral assemblage with rare earth element abundances distinct from described lunar material and more similar to chondrules found in ordinary or carbonaceous chondrites. Its discovery and description is significant for understanding the composition of lunar impactors. Previously, the main evidence for chondritic lunar impactors was from chondritic relative abundances and near chondritic ratios of highly siderophile elements in lunar impact melt breccias. However, the presence of this clast, along with two other chondritic clasts from Apollo soils 12037 and 15602, provides clues to the identity of ancient meteorite impactors on the Moon. (c) 2006 Elsevier Inc. All rights reserved.